From 1863 to 1867 Sanborn was an editor of the Boston Commonwealth, from 1867 to 1897 of the Journal of Social Science, and from 1868 to 1914 a correspondent of the Springfield Republican.
He contributed largely to the Proceedings of the Massachusetts Historical Society (1903–15). He also edited two volumes of Theodore Parker's Writings (1914), introduced Newton's Lincoln and Herndon (1913), and wrote brief biographies of Samuel Langdon (president of Harvard College), of Ellery Channing and of Mrs. Abbott-Wood of Lowell. He edited for the Boston Bibliophile Society five volumes of Thoreau's manuscripts, a volume of the Shelley-Payne correspondence, and one of the Fragments and Letters of T. L. Peacock. He edited writings of Paul Jones.